Steroid hormones are potent modulators of transcriptional events that together regulate the complex processes associated with differentiation homeostasis and development. The mechanism of action of these molecules is related in that the effector molecule binds to a specific intracellular receptor. This binding alters the structure of the receptor, thus increasing its affinity for specific recognition sites within the regulatory region of target genes. In this way, the steroid directs a program of events that leads to a change in cell phenotype.
Steroid hormones, thyroid hormones and certain vitamins can regulate cellular differentiation morphogenesis and homeostasis by binding to specific intracellular receptor proteins. Ligand activated receptor complexes are capable of activating or repressing transcription of a specific set of target genes. Thus, the receptor proteins are capable of reprogramming cellular function at the genomic level in response to hormonal or vitamin signals.
The isolation and sequencing of the DNA clones encoding these receptor proteins provided evidence that the receptors comprise a family of closely related genes. Additional members of this family have been identified on the basis of amino acid sequence homologies appearing in three regions of the proteins. The C.sub.1 region contains a highly conserved DNA binding domain, while the consensus regions C.sub.2 and C.sub.3 are located in the ligand binding domain of the receptors. Approximately 20 new members of this gene family have been identified. The presence of consensus regions C.sub.2 and C.sub.3 has lead to speculation that the new members of this family are also ligand activated transcription factors. However, since no ligand has been identified which directly activates any of the new members of this family, they are "orphan receptors" in search of a ligand. A limited number of compounds have been identified which can activate the specific orphan receptors indirectly. However, the mechanism by which these compounds activate the respective orphan receptors and the physiological relevance of these activators is unclear.
For several years it has been accepted dogma that steroid receptors can be activated only by their specific cognate ligands. Further it has been thought that chemical signals which bind to membrane receptors exert their biological effects through pathways which are separate and independent of the steroid receptor pathways.
The present invention shows that there is cross-talk between membrane receptors and steroid receptors. Further, there is provided a new system for detecting the ability of a wide range of compounds to bind membrane receptors, alter intracellular phosphorylation and to activate transcription by steroid, vitamin, orphan and chimeric receptors. This system and the method of using the system in an assay demonstrate that the binding to membrane receptors is coupled to a phosphorylation event which activates steroid receptors and regulates transcription.